Ozone and by-products generation characteristics by novel air-fed ozone generator which combines homogeneous discharge and filamentary discharge

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1 nd International Symposium on Plasma Chemistry July 5-, 15; Antwerp, Belgium Ozone and by-products generation characteristics by novel air-fed ozone generator which combines homogeneous discharge and filamentary discharge N. Osawa and Y. Yoshioka Kanazawa Institute of Technology, 7-1 Ohgigaoka, Nonoichi, Ishikawa, Japan Abstract: Ozone and by-products generation characteristics from an air-fed ozone generator by filamentary discharge (FD), homogeneous discharge (APTD), and alternate (FD and APTD) discharge modes were investigated. The ozone yields by the alternate mode and the were 7.8% and 35.% lower than that of by. N O generation rates by the alternate mode and the were.4% and. % lower than that of by. The lowest N O concentration at the same ozone concentration was achieved by the alternate mode. Keywords: Ozone, low by-products emission, atmospheric pressure Townsend discharge. 1. Introduction Ozone is a strong oxidizing agent and it can be applied to water treatment, gaseous pollution control, etc [1]. Usually, ozone is produced by Dielectric Barrier Discharge (DBD), which is composed of many Filamentary micro-discharges (FDs). The reduced electric field strength (E/n) at the streamer head of FD was calculated as 0 Td in atmospheric pressure dry air by Komuro et al []. Electrons accelerated by this high E/n induce not only O dissociation but also N excitation and dissociation. Therefore, it is considered that if we use air as a source gas of ozone generation, by-products like N O, NO, NO, HNO 3, N O 5 would be generated [1]. So far, we succeeded in generating the Atmospheric Pressure Townsend Discharge (APTD) in dry-air [3] and investigated the application of the APTD to an air-fed ozone generator [4]. The experimental results showed that (1) the maximum ozone yield was obtained by the FD mode, however, the ozone yield decreased drastically at higher Specific Input Energy (SIE), () in case of APTD mode, the yield decreased slightly with the increase of SIE. Recently, we investigated by-products from two kinds of air-fed ozone generators using a Fourier Transform Infrared (FTIR) spectrometer with a multi path gas cell [5]. In the both types of ozone generators, HNO 3, N O 5, and N O were detected as by-products. However, intensities of the absorbance spectra of HNO 3, N O 5 and N O in ozone gas from were lower than those of from at the same ozone concentration. Therefore, we concluded that the APTD can suppress byproducts generation from the air-fed ozone generator. The reason seems to be as follows. Because the E/n of the APTD is lower than that of the streamer head, the dissociation of N molecule and water vapour and excitation of N molecule by electron impact are weak. This leads to the suppression of subsequent NOx generation. Recently, we succeeded in generating the alternate mode of APTD and FD by a simple DBD device in dry-air at atmospheric pressure [6]. We considered if we apply this mode to the air-fed ozone generator, ozone with low by-products can be generated. In this paper, we investigated ozone and by-products generation characteristics by alternate mode DBD.. Experimental setup and discharge appearance.1. Experimental setup Fig. 1 shows experimental setup. This system consists of an AC high voltage power source, a DBD device set in a chamber and various measurement devices. Atmospheric dry-air (absolute humidity: mg/m 3 ) was used as source gas of ozone generation. The flow rate was fixed to.0 L/min (5 C, 13 Pa) using a mass flow controller (SEC-0mk3, Horiba, Ltd.). Concentrations of ozone and N O were measured by an UV absorption type ozone monitor (EG-00B/01, Ebara Jitsugyo Co., Ltd.) and the FTIR spectrometer (IR Affinity-1, Shimadzu corp.) with the multi path gas cell (3 m, Gemini Scientific Instruments) respectively. Gas temperature in a plasma zone was measured directory by a fibre optic thermometer (FL-00, Anritsu Meter Co., Ltd.). AC high voltage was applied to the DBD device by a step-up transformer. The maximum applied voltage and frequency were. kvp (zero-to-peak voltage) and 0 Hz respectively. The applied voltage (V) and the current were measured by an oscilloscope (TDS-4B, Tektronix, Inc.) using a high voltage probe (EP-K, AC Power Source ~0V ~1.1kHz Step-up transformer (1:1) H.V. probe (00:1) Dry air Integral capacitor (0.1 μf) Ch.1 Ch. Ch.3 Oscilloscope (0 MHz,.0 GS/s) MFC Shunt resistor ( kω) Differential probe (0 MHz) Electrode Gas out Fig. 1. Experimental system. Pressure gauge Barrier Chamber FTIR with gas cell Ozone monitor Image intensifire Digital camera fibre optic thermometer O-3-4 1

2 R15 0 Spacer ( mm) H.V. applied voltage P = 0.1 MPa f = Hz HV: A4 LV: A473 gap voltage 1 0 Gap length 1 Discharge zone Solder H.V. L.V. Voltage (kv) 0 - current 0-1 Current (ma) Electrode [tungsten] (Thickness: 0.01 mm) Exhaust hole φ φ Fig.. DBD device Barrier [AlO3: 9%] (A4 or A473) Table 1. Features of barrier material. Material code A473 A4 Discharge mode APTD FD Main material Al O 3 Purity 9% Relative permittivity (1 MHz) Color White Black Surface roughness R a µm 0.4 µm Table. Configurations of DBD device. Discharge mode Barrier material H.V. side L.V. side FD A4 A4 APTD A473 A473 alternate mode (APTD and FD) A473 A4 Pulse Electronic Engineering Co., Ltd.) and a differential probe (70094, Yokogawa Electric Corporation) respectively. An integral of the current (charge q) was measured from the voltage drop across an integral capacitor (0.1 μf). Besides, the discharge power was calculated by multiplying the area of V q Lissajous figure by power frequency. Discharge photographs were taken by a digital camera (D0E, Nikon Imaging Japan Inc.) with an image intensifier (C, Hamamatsu Photonics K. K.). Fig. shows a DBD device. We used two kinds of alumina barriers (Material code: A473 and A4, Kyocera Corporation), one of which can generate APTD and the other cannot. Features of barrier materials are summarized in Table 1. The gap length was fixed to mm using spacers. The size and thickness of the barrier are 0 cm and mm respectively. The electrode material is tungsten, and its effective area and thickness are 58.9 cm and 0.01 mm respectively. The electrode was implanted into alumina barrier in order to avoid generation of abnormal discharges from the edges of the electrodes. Therefore, the barrier thickness from the tungsten film electrode to the barrier surface is 1 mm. Configurations of DBD device were summarized in Table. When the A473 alumina and A4 alumina were used as H.V. side and L.V. side barrier material respectively, APTD and FD generates alternately in every half cycle in the discharge volume. L.V Time (ms) Fig. 3. Current, gap voltage, and applied voltage waveform of alternate mode of APTD and FD. (a) positive polarity (b) negative polarity Fig. 4. Discharge photographs... Discharge appearance of alternate mode Since discharge appearance of APTD in dry air was reported in our paper [3], we will introduce the discharge photographs and current waveforms of the alternate mode of APTD and FD [6]. Fig. 3 shows the waveforms of the current, the applied voltage and the gap voltage. If A473 alumina barrier becomes cathode (0 ms), the current waveform is smooth without pulses. However, if A4 alumina barrier becomes cathode ( 15 ms), the current waveform has many pulses as in typical filamentary DBD. Fig. 4 shows discharge photographs of the alternate mode. If A473 alumina barrier becomes cathode, FDs are not recognized in the gap. The luminosity gradually increased from cathode to anode, which is a feature of APTD. However, if A4 alumina barrier becomes cathode, many FDs were recognized in the gap. From these photographs, it is apparent that and FD mode were generated alternately by the simple DBD device. Next, the ozone and N O generation characteristics by,, and alternate mode were investigated. 3. Experimental results Fig. 5 shows the ozone concentration as a function of SIE in 3 different discharge modes. The ozone concentrations by, and alternate mode increased with the increase of SIE. However, in cases of and alternate mode, saturation tendency appeared at high SIE region of around 0 J/L. In the region of SIE below 4 J/L, highest ozone concentration was obtained by at the same SIE. Fig. 6 shows the ozone yields as a function of SIE in O-3-4

3 00 Ozone concentration (ppm) Fig. 5. Ozone concentration. N O concentration (ppm) Fig. 7. N O concentration. Ozone yield (g/kwh) Generation rate of N O (g/kwh) Fig. 6. Ozone yield. different discharge modes. The highest ozone yield was obtained by at the SIE of around J/L. However, it decreased drastically with the further increase of SIE. In case of, the maximum ozone yield was obtained less than the SIE of J/L, and the yield was 1.8 times lower than that by. However, the ozone yield did not decrease with the increase of SIE. In case of alternate mode, the highest ozone yield was obtained at the SIE of around J/L. However, the yield was 1. times lower than that by. The ozone yield did not decrease up to 0 J/L of SIE as in the. However, the yield decreased drastically with the further increase of SIE as in the. Fig. 7 shows the N O concentration as a function of SIE in 3 different discharge modes. The N O concentrations by, and alternate mode increased with the increase of SIE. The lowest N O concentration at the same SIE was obtained by APTD mode. Fig. 8 shows the generation rate of N O as a function of SIE in 3 different discharge modes. In case of, the N O generation rate increased with the increase of SIE and it reached the maximum value of around 0 J/L. Then, the generation rate decreased with the further increase of SIE. In case of, the generation rate increased with the increase of SIE up to J/L. However in the region between J/L and 0 J/L, the increase of generation rate was small. In case of alternate mode, generation rate was times higher than that N O concentration (ppm) Fig. 8. Generation rate of N O Ozone concentration (ppm) Fig. 9. Ozone concentration vs. N O concentration by. However, the tendency was alike as in. Fig. 9 shows the relation between N O concentration and ozone concentration in 3 different discharge modes. It can be seen that the lowest N O concentration at the same ozone concentration was achieved by the alternate mode. 4. Discussions Here, we discuss why ozone yield were influenced by the discharge mode. Generally, the thermal decomposition of ozone starts at a gas temperature of above 1 C [7] and this reaction is represented as follows, O 3 + thermal O + O (R1) In order to clarify the influence of the heat by O-3-4 3

4 Plasma zone gas temperature ( O C) Decrease of ozone yield (%) Decrease of N O generation rate (%) Fig.. Plasma zone temperature Ave. = 35.% Ave. = 7.8% Ave. =.% Ave. =.4% Fig. 11. Decrease of ozone yield and generation rate of N O (Reference value: ) discharge, we measured plasma zone gas temperature using the fibre optic thermometer. Fig. shows plasma zone gas temperature as a function of SIE in 3 different discharge modes. The temperatures increased with the increase of SIE. However, the temperatures did not influenced by discharge modes. Therefore, the gas temperature is not the cause of the difference of ozone yield by discharge mode. Next, we discuss why lowest N O concentration at the same ozone concentration was obtained by the alternate mode. Fig. 11 shows the decrease of ozone yield and the decrease of N O generation rate, which were calculated by using the experimental data from as reference value. Average values of the decrease of ozone yield by alternate mode and by were 7.8% and 35.% respectively. On the other hand, average values of the decrease of N O generation rate by alternate mode and by were.4% and. % respectively. From these calculation results, we concluded that, since decrease of ozone yield was lower than that of N O generation rate, lowest N O concentration at the same ozone concentration was obtained by the alternate mode. Finally, we discuss why the decrease of N O generation rate was higher than that of ozone yield in case of alternate mode. It is well known that ozone is formed by these reactions, e + O e + O (A 3 + u ) e + O( 3 P) + O( 3 P) (R) e + O e + O (B 3 - u ) e + O( 3 D) + O( 3 P) (R3) O + O + M O 3 + M (R4) Here, the average electron energy of about 6 9 ev would be ideal for the dissociation of O by electron impact [1], because the energy thresholds for reactions of (R1) and (R) are 6.0 ev and 8.4 ev, respectively. On the other hand, it is reported that N O is formed by N metastable reaction [8], e + N e + N (A 3 u + ) (R5) e + N e + N (B 3 Π g ) (R6) N (A, B) + O N O + O (R7) Here, the energy thresholds for reactions of (R5) and (R6) are 6.17 ev and 7.35 ev, respectively [9]. Form these reactions, it is apparent that the energy threshold of N (A 3 u + ) generation is bit higher than that of O (A 3 u + ) generation. Therefore, decrease of N O generation rate is higher than that of ozone yield. 5. Conclusions The effects of discharge mode to ozone and by-products generation were investigated. The conclusions are as follows; (1) Average ozone yields by alternate mode and by APTD mode were 7.8% and 35.% lower than that of by FD mode () Average N O generation rates by alternate mode and by were.4% and. % lower than that of by. (3) In case of alternate mode, since the decrease of N O generation rate is higher than that of ozone yield, the low N O emission performance at the same ozone concentration was achieved. 6. Acknowledgements This work was supported by a MEXT (Ministry of Education, Culture, Sports, Science and Technology) Supported Program for the Strategic Research Foundation at Private Universities 11 16, a Grant in Aid for Young Scientists (B), 58111, and Takahashi Industrial and Economic Foundation. We would like to thank Kyocera Corporation for providing alumina barrier plates. 7. References [1] B. Eliasson, U. Kogelschatz, IEEE Trans. Plasma Sci., 19, 6 (1991). [] A. Komuro, R. Ono, J. Phys. D: Appl. Phys., 47 (14). [3] N. Osawa, Y. Yoshioka, IEEE Trans. Plasma Sci.,, 1 (1). [4] N. Osawa, H. Kaga, Y. Fukuda, S. Harada, Y. Yoshioka, R. Hanaoka, Eur. Phy. J Appl. Phys., 55 (11). [5] N. Osawa, and Y. Yoshioka, J. Adv. Oxid. Technol., 17, (14) [6] N. Osawa, Y. Yoshiok, in Proc. of the 14 th Int. Symp. on High Pressure Low Temperature Plasma Chem. (HAKONE XIV) (14) [7] M. Taguchi, IEEJ Trans. FM, 134, 11 (14) [8] B. Eliasson, U. Kogelschatz, IEEE Trans. Plasma Sci., 19, (1991) 4 O-3-4

5 [9] Y. Itikata, J. Chem. Ref. Data, 35, 1 (06) O-3-4 5